A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion”, respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist) or a metrology or inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.
The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g. having a wavelength of 365, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength in the range of 5–20 nm), as well as particle beams, such as ion beams or electron beams.
The term “patterning device” used herein should be broadly interpreted as referring to devices that can be used to impart a projection beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the projection beam may not exactly correspond to the desired pattern in the target portion of the substrate. Generally, the pattern imparted to the projection beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.
Patterning devices may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions; in this manner, the reflected beam is patterned.
The support structure supports, i.e. bears the weight of, the patterning device. It holds the patterning device in a way depending on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support can be using mechanical clamping, vacuum, or other clamping techniques, for example electrostatic clamping under vacuum conditions. The support structure may be a frame or a table, for example, which may be fixed or movable as required and which may ensure that the patterning device is at a desired position, for example with respect to the projection system. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term “patterning device.”
The term “projection system” used herein should be broadly interpreted as encompassing various types of projection system, including refractive optical systems, reflective optical systems, and catadioptric optical systems, as appropriate for example for the exposure radiation being used, or for other factors such as the use of an immersion fluid or the use of a vacuum. Any use of the term “lens” herein may be considered as synonymous with the more general term “projection system”.
The illumination system may also encompass various types of optical components, including refractive, reflective, and catadioptric optical components for directing, shaping, or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens.”
The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables). In such “multiple stage” machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure.
The lithographic apparatus may also be of a type wherein the substrate is immersed in a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the final element of the projection system and the substrate. Immersion liquids may also be applied to other spaces in the lithographic apparatus, for example, between the mask and the first element of the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems.
In order to allow accurate imaging of the mask to the substrate, the light from a radiation source is focused in a pupil plane by an illuminator, to allow optimal illumination of the patterning, as will be discussed in more detail below. The illuminator establishes a predetermined light distribution in the pupil plane, using among other things a diffractive optical element. Such light distributions in the pupil plane are referred to as illumination modi. Examples of illumination modi are: conventional illumination corresponding to an on-axis disk-shaped rotationally symmetric pupil distribution, annular illumination corresponding to a ring-shaped pupil distribution, dipole illumination corresponding to two off-axis poles, or quadrupole illumination corresponding to four off-axis poles. A recent development is the use of a so called low sigma illumination mode, which is similar to conventional illumination except for the radius of the disk-shaped (or top-hat) distribution which is relatively small.
In order to ensure accurate imaging of the patterning device on the substrate, in the pupil plane the light distribution in the x direction is preferably equal to the light distribution in the y direction (x and y taken perpendicular to the optical axis z, and perpendicular with respect to each other).
Often a laser source is used as a radiation source for generating the electromagnetic radiation needed. Such laser sources usually produce a relatively narrow, collimated laser beam, with a relatively small divergence. It is however observed that the divergence in the x direction is often different than the divergence in the y direction. Especially when a beam expander is used to expand the laser beam to a beam with a convenient cross-section, and the magnification in the x-direction differs from that in the y-direction, then the divergence in the x-direction may become even more different than the divergence in the y-direction. The divergence of the beam results in a spread out of the light in the pupil plane, i.e. the pattern in the image plane is somewhat blurred with respect to the pattern that would have been produced by an ideally collimated light beam.
Further, since the divergence in the x direction is different from the divergence in the y direction, this also results in an uneven light distribution in the pupil plane. If, for instance, a ring shaped illumination mode is chosen, the difference in the x and y divergence of the laser beam results in different intensity profiles in the pupil plane in the x and y direction. Not only the shape of the intensity profile will differ, but also the absolute intensities between corresponding positions in the x and y profile.
This has negative effects on the projection of the patterning device on the substrate as will be understood by a person skilled in the art. It might for instance result in uneven imaging results on the substrate for pattern lines elongated in both the x and y direction.
U.S. Pat. No. 6,583,855 B2 in the name of the applicant describes several solutions for solving anomalies of intensity distribution in a plane perpendicular to the optical axis of the radiation system or the projection system. The document describes to counteract anomalies by changing the orientation of a diffractive optical element, which is normally positioned in between the laser source and the pupil plane, as will be explained in further detail below. According to an alternative solution anomalies are counteracted by forming the diffractive optical element of non-regular hexagonal microlenses, stretched along one axis to introduce a predetermined ellipticity error to correct for ellipticity errors caused by other elements.
U.S. Pat. No. 6,583,855 B2 further describes other solutions to the identified problem, such as using a filter in the path of the projection beam, which is partially transmissive to radiation of the projection beam, where said partially transmissive filter has a transmission distribution which counteracts said anomalies in the pupil plane.
The disadvantage of the solutions presented in U.S. Pat. No. 6,583,855 B2 is that all need to be arranged and adjusted specially for a specific source. The adjustments made to the diffractive optical element need to be specifically designed for a certain radiation source of which the difference in divergence between the x and y is known. Also, the use of a transmissive filter requires special adjustment to one specific radiation source.